In the semiconductor industry, it is desirable to obtain temperature uniformity in the substrate during temperature cycling of substrate. Temperature uniformity provides uniform process variables on the substrate (e.g. layer thickness, resistivity, etch depth) for temperature activated steps such as film deposition, oxide growth and etching. In addition, temperature uniformity in the substrate is necessary to prevent thermal stress-induced wafer damage such as warpage, defect generation and slip.
FIG. 1 schematically shows a prior art flood type rapid thermal heating apparatus in which a wafer 11 disposed in chamber 12 is heated by radiation from a plurality of lamps 13. This type of heating apparatus does not provide adequate spatial control of temperature. The primary difficulty is that different areas on the wafer may have different energy absorption or emissivity characteristics. For example, if a flood heating source (assuming uniform illumination across the wafer) is used to heat a wafer during a rapid thermal processing cycle in which the thermally-isolated wafer may be ramped in temperature on the order of 10.degree.-300.degree. C./sec, the edge will maintain a different temperature than the middle because the edge can accept radiant energy from or emit radiant energy to a wider field of view. FIG. 2 shows the temperature at the center and edges of a wafer as a function of time for a flood type heating source During the temperature ramp-up portions of the heating cycle the edges will be hotter than the center while during the steady state and ramped-down portions the edges will be cooler than the center. These edge to center temperature differences create radial stresses in a wafer which if large enough, can damage the wafer, and are not tolerable in many processes, especially high temperature processes in which the mechanical strength of the wafer is substantially reduced. For example, at 1150.degree. C. the center to edge temperature difference on a four inch silicon wafer of approximately 5.degree. C. can induce dislocation formation and slip. Some conventional flood heating sources, such as a bank of tungsten-halogen lamps or a single arc lamp, can be modified to compensate for center-to-edge temperature differences of one sign, for example, during the temperature ramp up. Shading or reflectors can be used to provide more light energy to the center of the wafer as compared to the edge, but it is impossible for such a heating source to provide temperature uniformity during all parts of the thermal cycle.
Temperature gradients can also be induced by other sources. For example, a wafer may have non-uniform emissivity because of spatial modifications to surface areas or volumes of the wafer. These modifications could include films which have been patterned by photo lithography or locally doped regions such as buried layers for bipolar transistors. In addition, the temperature gradients can be induced by localized gas cooling or heating effects, as well as non-uniform endothermic or exothermic reactions which may occur on the substrate surface during processing.